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Bacon and cheesecake can alter the brain in ways similar to heroin and cocaine, according to scientists who say they have found the most compelling proof yet that high-fat foods rewire the brain and drive the development of compulsive eating.

When rats raised on regular chow were suddenly given unrestricted access to a high-fat diet, they lost complete control over their eating. Not even mild foot shocks kept them from compulsively feasting on chocolate bars, cream-stuffed cakes, sausage, frosting and other highly palatable human foods. Within 40 days, their body weight had increased 25 per cent.

The rats not only got fat, they also showed addiction-like changes in brain reward circuits - the same changes that have been reported in humans addicted to drugs.

Specifically, the obese rats showed lower levels of a receptor in the brain called the dopamine D2 receptor. The D2 receptor responds to dopamine, the chemical associated with feelings of reward. The brain releases bursts of dopamine when we eat food that tastes good.

The more junk food the rats ate, the more they overloaded the brain's reward circuitries until they essentially crashed. As the pleasure centres in the brain became more and more blase, and less responsive, the rats quickly turned into compulsive overeaters. They were motivated to keep eating to get their fix.

"They're in a state of reward deficit, so that they're now even more motivated to obtain rewarding food, perpetuating this vicious cycle even further," said study co-author Paul Kenny, an associate professor at Scripps Research Institute in Florida.

The lowered D2 receptor levels - a side effect of overeating high-fat food - also seemed to drive the animals to develop "habitual" feeding behaviours that made them "less able to shift their dietary preferences," Kenny says.

When the researchers took the high-fat foods away, leaving only the healthy, but boring chow - what the scientists dubbed the "salad bar option" - the rodents essentially voluntarily starved themselves.

"They liked the junk food so much they would rather starve than shift onto the regular chow," Kenny said. Even after two weeks of having no junk food, "they still hadn't returned to the level of intake that you see in the control animals for the standard chow. That goes to show just how powerful this food was."

When they artificially knocked down the dopamine receptor using a special virus, nothing happened when rats were given regular chow. They didn't become compulsive in any way, Kenny said. "Their brain reward systems looked fine.

"But the second you gave it palatable food, it showed very rapidly these addiction-like changes."

Some people may be born with a predisposition to have lower D2 levels.

"That may be why they're more likely to gain weight. They're already halfway down that road, if you will," Kenny said.

The findings, published Sunday in an advance online edition of the journal Nature Neuroscience, could have profound implications for the millions of Canadians struggling to control their eating.

"What this is telling you is that, if you persist in eating food that you know is bad for you, there is a chance that you will develop a habit, and you will keep on going back to that food unless you make a really strong, conscious effort to stop it," Kenny said.

"It's incumbent upon people to make sure that they're more respectful and aware of what they're eating. Just be aware that there are dangers and risks associated. Enjoy (high-fat) food but make sure it's occasionally and very-well controlled. Don't overindulge repeatedly, because there could be repercussions."

Dr. Valerie Taylor, an assistant professor in psychiatry and behavioural neuroscience at McMaster University in Hamilton, said the study is a validation "that some people are simply more vulnerable to the whole concept of being addicted to food."

"The fact that we're now in this high-temptation environment further serves to exacerbate that."

Taylor said the study provides "very strong evidence supporting what a lot of us who work in the field have seen clinically - that, for some people, it's more than just simply willpower. There's something else going on."

According to the latest estimates from Statistics Canada, 37 per cent of the adult population age 20 to 69 - 7.9 million people - are overweight. Another 24 per cent - 5.3 million - are obese.

The new study is part of a growing body of research into the "hedonic mechanisms" contributing to obesity. The preliminary findings captured headlines in October when an abstract presented at a neuroscience meeting in Chicago reported that junk food binge eating is hard to stop.

The final report goes further, and explains just what's happening in the brain.

Three groups of rats were studied. In addition to unlimited access to their regular chow, one group was given one hour of access a day to the junk food, while another group had 18 to 23 hours of access each day, for 40 consecutive days.

Rats that had one hour access to the junk food binge-ate, gorging on the food during those one-hour sessions, so much so that they consumed almost two- thirds of their daily calories in that one-hour session. "But they didn't gain weight, and they didn't show those addiction-like changes," Kenny said.

Rats given unlimited access to the sausages, frostings and cakes didn't binge or gorge, but they snacked all day. They kept eating, consuming twice as many calories as the "control" rats, even when the flashing cue light came on that was paired with a foot shock.

"Many drug addicts know that what they're doing is bad - they're damaging their health, their finances, their family. But they find it very difficult to stop - the behaviour is almost beyond their control," Kenny said.

"The same thing happened here: The animals kept on eating, even when there was something in the environment that said something bad was going to happen. They simply ignored it, and they just kept on eating."

Kenny said treatments known to work for drug addictions may be effective for people who overeat junk food.

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